Programmable bandpass analog to digital converter based on error feedback architecture
A delta sigma modulator based analog to digital converter is presented, having a first quantizer and a digital error feedback system comprising a second quantizer and a digital bandpass noise shaping system. The second quantizer provides a second quantized output to the noise shaping system according to the first quantized output, the system analog input, and the noise shaped feedback signal. The digital noise shaping system provides a feedback signal to the first quantizer according to the second quantized output, where the feedback signal is bandpass noise shaped with respect to a quantization error of the first quantizer.
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The present invention relates generally to analog to digital conversion systems and more particularly to bandpass analog to digital converters and programmable bandpass delta sigma modulators therefor having digital error feedback noise shaping systems.
BACKGROUND OF THE INVENTIONAnalog-to-digital (A/D) converters are electrical circuit devices that convert continuous signals, such as voltages or currents, from the analog domain to the digital domain where the signals are represented by numbers. A variety of A/D converter types exist, including flash A/Ds, sub-ranging A/Ds, successive approximation A/Ds, and integrating A/Ds. Another type is known as a sigma delta or delta sigma (e.g., Δ-Σ) A/D converter that includes a delta sigma modulator operating as a type of noise shaping encoder, typically with a 1-3 bit quantized digital output. Delta sigma or sigma delta modulators are often used in mixed signal integrated A/D converters, because of their insensitivity to CMOS process linearity and matching problems when compared to other A/D converter types. These features make delta sigma based mixed signal solutions very attractive for a number of applications, such as audio, receiver channels of communication devices (wireless in particular), sensor interface circuits, and measurement systems.
Delta sigma converters are operated at a significantly higher sampling rate than the bandwidth of the analog input signal, a technique referred to as oversampling, wherein the analog input signal is sampled at a very high sampling rate in order to perform a noise shaping function. The oversampling is commonly performed at a multiple of the Nyquist rate (FN) for a given input signal frequency content (e.g., sampling frequency FS is often 10 to 1000 times FN), wherein quantization noise power is spread over a bandwidth equal to the sampling frequency, thereby reducing the noise density in the band of interest. A noise shaping or loop filter, typically a lowpass filter (e.g., integrator), is commonly provided in the forward signal path of the delta sigma modulator to push some of the quantization noise into the higher frequency spectrum beyond the band of interest. Digital filtering is performed on the oversampled digital output to achieve a high resolution, and decimation is employed to reduce the effective sampling rate back to the “Nyquist” rate.
In addition to traditional lowpass loop filters, delta sigma modulators often include bandpass loop filters where a signal of interest lies within a certain frequency band. For example, in wireless communications devices, bandpass delta sigma A/D converters are employed to ascertain a signal of interest in narrow or wide bands (e.g., Global Systems for Mobile communications (GSM), Code Division Multiple Access (CDMA), Wideband Code Division Multiple Access (WCDMA), etc.) using a bandpass loop filter to reduce noise outside the particular band of interest. In this regard, GSM applications typically involve channel bandwidths of 200 kHz, the Bluetooth CDMA standard corresponds to the 600 kHz and 1.0 MHz bandwidths, WCDMA and Bluetooth standards correspond to 2.0 and 3.0 MHz bandwidths, with video standards using even wider bands. Conventional delta sigma modulation based bandpass A/D converters implement the noise shaping transfer function for the loop filter using analog components, including inductors and other highly nonlinear circuit elements. These circuits, once constructed, are optimized for a fixed bandwidth and a fixed sampling frequency FS, typically about 4 times the center frequency of the band of interest (e.g., the center frequency is about ¼ of FS).
However, in certain receiver applications, different channels are located at different center frequencies. For example, switching from one channel to another in a GSM receiver requires changing the center frequency of the A/D converter loop filter. In conventional designs, this has been accomplished using a variable sampling clock frequency to address different frequency bands. Variable clocks, however, require complex phase locked loop (PLL) circuitry, thus increasing the circuit area occupied by the converter and the cost. In other situations, it is desirable to create a bandpass delta sigma modulator that may be used in both narrow band and wider band applications. For instance, the same bandpass A/D converter may need to convert signals in one or more narrow GSM bands, as well as in wider bands for CDMA and WCDMA applications. Conventional designs do not allow this flexible operation, since the analog bandpass loop filter poles and zeros (e.g., and hence the passband width) are fixed, making it difficult to address multiple application standards. Furthermore, analog bandpass filters for such delta sigma modulator applications include highly non-linear components (e.g., inductors, etc.), which are difficult to implement in CMOS fabrication processes and integrators with stability shortcomings, high signal swings, high area usage, and high power consumption. Accordingly, there is a need for improved bandpass delta sigma modulators and A/D converters to allow operation with different bandwidths and center frequencies.
SUMMARY OF THE INVENTIONThe following presents a simplified summary in order to provide a basic understanding of one or more aspects of the invention. This summary is not an extensive overview of the invention, and is neither intended to identify key or critical elements of the invention, nor to delineate the scope thereof. Rather, the primary purpose of the summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. The invention relates to bandpass A/D converters and delta sigma modulators wherein the bandpass noise shaping or filtering is performed digitally, which facilitates programmability with respect to bandwidth and center frequency using simple clock circuitry, with low power consumption, reduced circuit size, and wherein the noise shaping circuitry can be fashioned from linear components.
In one aspect of the invention, delta sigma based analog to digital conversion systems are provided, in which digital error feedback is used to perform noise shaping, such as digital bandpass filtering. The modulator comprises a first quantizer and a digital error feedback system, where the first quantizer provides a digital output based on the system analog input and a noise shaped feedback signal from the digital system. The error feedback system provides noise shaping, for example, digital bandpass filtering with programmable filter poles and zeros to address multiple applications, where the center frequency and/or bandwidth can be easily changed without complex PLL circuits and without nonlinear analog components. In one implementation illustrated below, the digital feedback system includes a high resolution second quantizer and a digital noise shaping system providing the noise shaped feedback signal. The second quantizer provides a digital representation of the quantization error of the first quantizer, which is then noise shaped (e.g., bandpass filtered) by the noise shaping system. The programmable noise shaping can be provided by a plurality of digital bandpass filter systems having different filter pole and zero locations, with a multiplexer providing the noise shaped output from a selected filter as the feedback signal.
The following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of only a few of many ways in which the principles of the invention may be employed.
One or more implementations of the present invention will now be described with reference to the attached drawings, wherein like reference numerals are used to refer to like elements throughout. The invention relates to analog to digital converters that may be employed in receiver circuits for wireless communications devices or other applications, wherein digital noise shaping is used to provide bandpass or other filtering.
Referring initially to
The conventional converter 10 provides a noise shaping transfer function using analog components, including amplifiers, inductors and other highly nonlinear circuit elements, where the filter 12 is designed for a fixed bandwidth and a fixed sampling frequency FS, typically about 80 MHz (e.g., 4 times the IF2 center frequency of 20 MHz). However, as discussed above, accommodating multiple channels in a GSM system requires the receiver 2 of
Referring initially to
The first quantized output Y(n) is then provided to a digital filter and decimation system 106 that filters Y(n) to produce a sequence of digital values (e.g., A/D converter output) DOUT. The system 106 reduces undesirable noise in the digital signal Y(n) and performs decimation by resampling the signal Y(n) at a lower rate to remove redundant signal information introduced by the oversampling process to provide the multi-bit system digital output DOUT representative of the system analog input. The digital decimation system 106 may be constructed using any digital logic and/or analog circuitry within the scope of the invention.
The exemplary first quantizer 104 comprises a 3-level flash A/D converter 104a (
The system 100 further comprises a second quantizer or A/D converter 112 and a digital noise shaping system 114 providing the feedback signal W(t) to the summer 102. The second quantizer 112 receives the signal U(t) and the converted (e.g., 3-level analog) signal Y(n) via a summer 110 as shown in
As the signal U(t) is the difference between the input and output of the first quantizer 104, the second quantized output U(m) is representative of the quantization error of the first quantizer 104. This converted quantization error signal U(m) is then noise shaped digitally in the system 114 and provided as the noise shaped feedback signal W(t). Other second quantizers 112 are possible within the scope of the invention, for example, having M levels, where M can be any positive integer, including M greater than 64. As discussed further below, the system 100 provides bandpass noise shaping with respect to the quantization error of the first quantizer 104, wherein the number of levels M of the second quantizer 112 sets the noise floor of the system 100. In this regard, higher order second quantizers 112 may advantageously lower the noise floor of the system 100.
As further illustrated in
As illustrated in
In accordance with an aspect of the invention, the filter poles and zeros of the digital bandpass filter system are variable or programmable, thereby allowing the bandwidth and/or the center frequency of the passband to be adjusted. This programmability facilitates selective adaptation of the A/D conversion system 100 to one or more different standards, such as GSM, CDMA, WCDMA, etc. for mobile communications applications, including selectively changing the passband center frequency (e.g., for GSM channel switching) and/or the bandwidth (e.g., for changing from GSM to CDMA or WCDMA applications). In this regard, it is noted that the digital noise shaping system 114 can be constructed using any digital circuitry, for example, using switched capacitor and comparator circuits to implement the third quantizer 128, the summing nodes 124 and 130, and the bandpass filter circuits (e.g., H(z)) 120 and 126. For example, the circuits 120 and 126 may be constructed to implement a difference equation wherein the difference equation coefficients are adjustable in order to change the filter pole and/or zero locations (e.g., and hence to change the passband center frequency and/or bandwidth). In one preferred implementation, moreover, the digital noise shaping system 114 provides adjustable pole and zero locations by including a plurality of digital bandpass filter systems having different (e.g., predetermined) pole and zero locations, with multiplexed outputs as illustrated further in FIG. 3D.
Referring also to
As illustrated in
Referring also to
Referring also to
The digital bandpass filter systems 114a-114j are coupled to receive the second quantized output U(m) and to provide corresponding bandpass noise shaping thereof. The systems 114a-114j create corresponding digital noise shaped feedback signals (e.g., 2-level signals in this example), which are provided as inputs to a selection circuit or multiplexer 115 in the digital system 114. As described above in connection with
The conversion system 100 can thus be employed to address multiple bands, for example, to switch between channels in GSM applications through selection of appropriately tuned digital bandpass filter systems 114a-114j using the multiplexer 115, while being operated with a fixed sampling clock (e.g., without requiring a variable PLL as in the conventional design of FIG. 1B). The system 100 also facilitates selection of narrow or wideband operation, for example, where certain of the filter systems 114a-114j can be designed for narrow bandwidths (e.g., 200 kHz) and others for wider bands (e.g., 1 MHz). Furthermore, because the noise shaping is performed digitally, the system 100 is constructed without amplifiers and inductors, thereby facilitating improved linearity, low power consumption, low voltage swings, and reduced circuit area.
As shown in
Y(z)=X(z)+(1−H(z))E1(z)−(1−H(z))E3(z)−H(z)E2(z),
where E1(z), E2(z), and E3(z) are the quantization errors of the first, second, and third quantizers 104a, 112a, and 128, respectively. Thus, the selected H(z) in the digital noise shaping system 114 provides bandpass noise shaping with respect to the quantization errors associated with the first and third quantizers 104a and 128, wherein the number of levels (e.g., 64) provided by the second quantizer 112a sets the noise floor of the system 100 in the selected passband. As can be seen in the noise shaping transfer function of equation 1, E1 and E3 are bandpass noise shaped by (1-H(z)). Furthermore, since E2 is associated with the high-level quantization of the second quantizer 112a, the noise floor within the selected passband can be tailored to meet a given performance specification (e.g., for GSM, CDMA, WCDMA, or other standard of interest).
Referring now to
The plot 200 in
Referring also to
Although the invention has been illustrated and described with respect to one or more implementations, alterations and/or modifications may be made to the illustrated examples without departing from the spirit and scope of the appended claims. In particular regard to the various functions performed by the above described components or structures (assemblies, devices, circuits, systems, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component or structure which performs the specified function of the described component (e.g., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary implementations of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”.
Claims
1. An analog to digital conversion system, comprising:
- a first quantizer providing a first quantized output responsive to a system analog input summed with an analog noise shaped feedback signal;
- a second quantizer and providing a second quantized output responsive to the summed analog version of the first quantized output, and the system analog input; and
- a digital noise shaping system responsive to the output of the second quantizers and providing the noise shaped feedback signal from the second quantized output.
2. An analog to digital conversion system, comprising:
- a first quantizer providing a first quantized output according to a system analog input and according to a noise shaped feedback signal;
- a second quantizer coupled with the first quantizer and providing a second quantized output according to the first quantized output, the system analog input, and the noise shaped feedback signal; and
- a digital noise shaping system coupled with the first and second quantizers and providing the noise shaped feedback signal according to the second quantized output, the noise shaped feedback signal being noise shaped by the digital noise shaping system with respect to a quantization error associated with the first quantizer;
- wherein the first quantizer is an N-level A/D converter providing the first quantized output having one of N discrete levels, wherein the second quantizer is an M-level A/D converter providing the second quantized output having one of M discrete levels, M and N being integers greater than 1, and wherein N is less than M.
3. The system of claim 2, wherein N is 3 or more and M is 64 or more.
4. The system of claim 1, wherein the second quantizer is an M-level A/D converter providing the second quantized output having one of M discrete levels, and wherein the number of levels M of the second quantizer determines a noise floor of the analog to digital conversion system.
5. The system of claim 1, wherein the first and second quantizers are flash A/D converters.
6. The system of claim 1, wherein the first and second quantizers individually comprise switched capacitor circuits.
7. The system of claim 1, wherein the digital noise shaping system comprises a switched capacitor circuit.
8. The system of claim 7, wherein the digital noise shaping system comprises a digital bandpass filter system.
9. The system of claim 1, wherein the digital noise shaping system comprises a digital bandpass filter system.
10. An analog to digital conversion system, comprising:
- a first quantizer providing a first quantized output according to a system analog input and according to a noise shaved feedback signal;
- a second quantizer coupled with the first quantizer and providing a second quantized output according to the first quantized output the system analog input, and the noise shaped feedback signal; and
- a digital noise shaping system coupled with the first and second quantizers and providing the noise shaped feedback signal according to the second quantized output, the noise shaped feedback signal being noise shaped by the digital noise shaping system with respect to a quantization error associated with the first quantizer;
- wherein the digital noise shaping system comprises a digital bandpass filter system;
- wherein the digital bandpass filter system comprises:
- a first digital bandpass filter coupled with the second quantizer and providing a first filtered output according to the second quantized output;
- a third quantizer coupled with the first and second quantizers and providing the noise shaped feedback signal according to the first filtered output and according to a filtered feedback; and
- a second digital bandpass filter coupled with the first digital bandpass filter and with the third quantizer, the second digital bandpass filter providing the filtered feedback according to the noise shaped feedback signal and according to the first filtered signal.
11. The system of claim 9, wherein the noise shaped feedback signal is noise shaped by the digital noise shaping system with respect to a quantization error associated with the first quantizer.
12. The system of claim 1, wherein the digital noise shaping system comprises:
- a plurality of digital bandpass filter systems coupled with the second quantizer, the digital bandpass filter systems having different filter pole and zero locations; and
- a multiplexer coupled between the digital bandpass filter systems and the first quantizer, the multiplexer providing the noise shaped feedback signal according to a selected one of the digital bandpass filter systems.
13. The system of claim 12, wherein the plurality of digital bandpass filter systems individually comprise:
- a first digital bandpass filter coupled with the second quantizer and providing a first filtered output according to the second quantized output;
- a third quantizer coupled with the first and second quantizers and providing the noise shaped feedback signal according to the first filtered output and according to a filtered feedback; and
- a second digital bandpass filter coupled with the first digital bandpass filter and with the third quantizer, the second digital bandpass filter providing the filtered feedback according to the noise shaped feedback signal and according to the first filtered signal.
14. The system of claim 13, wherein the noise shaped feedback signal is noise shaped by the digital noise shaping system with respect to a quantization error associated with the first quantizer.
15. The system of claim 1, wherein the noise shaped feedback signal is noise shaped by the digital noise shaping system with respect to a quantization error associated with the first quantizer.
16. A bandpass delta sigma modulator, comprising:
- a first quantizer providing a first quantized output responsive to a system analog input and a noise shaped feedback signal; and
- a digital error feedback system coupled to the first quantizer and providing the noise shaped feedback signal responsive to the first quantized output and including a second quantizer coupled to the first quantizer and providing a second quantized output responsive to the first quantized output, the system analog input, and the noise shaped feedback signal; and
- a digital noise shaping system coupled to the first and second quantizers and providing the noise shaped feedback signal.
17. The bandpass delta sigma modulator of claim 16, wherein the digital noise shaping system comprises a digital bandpass filter system.
18. The bandpass delta sigma modulator of claim 17, wherein the digital bandpass filter system comprises programmable poles and zeros.
19. The bandpass delta sigma modulator of claim 17, wherein the digital bandpass filter system comprises:
- a first digital bandpass filter coupled with the second quantizer and providing a first filtered output according to the second quantized output;
- a third quantizer coupled with the first and second quantizers and providing the noise shaped feedback signal according to the first filtered output and according to a filtered feedback; and
- a second digital bandpass filter coupled with the first digital bandpass filter and with the third quantizer, the second digital bandpass filter providing the filtered feedback according to the noise shaped feedback signal and according to the first filtered signal.
20. The bandpass delta sigma modulator of claim 17, wherein the noise shaped feedback signal is noise shaped by the digital noise shaping system with respect to a quantization error associated with the first quantizer.
21. The bandpass delta sigma modulator of claim 17, wherein the digital noise shaping system comprises:
- a plurality of digital bandpass filter systems coupled with the second quantizer, the digital bandpass filter systems having different filter pole and zero locations; and
- a multiplexer coupled between the digital bandpass filter systems and the first quantizer, the multiplexer providing the noise shaped feedback signal according to a selected one of the digital bandpass filter systems.
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Type: Grant
Filed: Dec 9, 2003
Date of Patent: Mar 8, 2005
Assignee: Texas Instruments Incorporated (Dallas, TX)
Inventor: Rahmi Hezar (Plano, TX)
Primary Examiner: Brian Young
Attorney: Wade James Brady, III
Application Number: 10/731,493